Remotely operated sensor packages have been used during the drilling of wells for a number of years. These packages are commonly found in drilling applications where information such as the inclination, azimuth, and various logging sensor measurements of the well are of interest.
During well drilling operations, drilling fluid, known in the art as drilling mud, is typically pumped down through the drill pipe and then through the drill bit to clean, lubricate, and cool the bit. The drilling fluid then returns to the surface by way of the annulus between the drill pipe and the bore hole or casing, where the drilling mud is cleaned of cuttings so that the drilling fluid can be re-used.
As early as 1942, it was established that the flowing drilling fluid could be used as a transmission medium for data developed down hole during drilling operations, thus the origin of the term “measuring while drilling”. To transmit information, a device was created that varied the pressure of the drilling fluid in the drill pipe by placing an orifice in the drill string and inserting a poppet into the orifice to form a “pulser”. By repeated insertion and removal of the poppet, a series of pressure increases was created in the drilling fluid that could be detected at the surface and used to convey information. Unfortunately, these pressure increases were of relatively low frequency, generally resulting in a pressure pulse with a rise time of 20-200 milliseconds, a duration of 0.25 to 3 seconds, and a fall time of 20-200 milliseconds. The resulting spectral content of the pulses created down hole was concentrated at frequencies below 20 Hz with the centroid of spectral energy below 3 Hz, and a peak energy centered in the range of 0.1 to 1.5 Hz.
In addition to severely limiting the data transmission rate, these low frequencies coincide with the noise frequencies generated during drilling. One common technique for improving the signal to noise ratio is to filter the noise. Unfortunately, conventional filtering, which is used to eliminate drilling noise, also removes much of the remaining energy from the transmitted pulse.
To overcome this shortcoming, the amplitude of the induced pressure pulses was increased. However, erosion of the poppet and orifice by the pressure pulses is a function of the imposed pressure drop. Thus, increasing the pressure drop decreased pulser life. Another problem with simply increasing the amplitude of the induced pressure pulses was the power required to create such pulses. The large power demand meant a large and more powerful prime mover to operate the poppet, and this meant greater weight and cost for the MWD system.
Therefore, it is an object and feature of this invention to provide a method of modifying the design of positive fluid pulsers that will shift the frequency of the signal away from the region of substantial drilling noise thereby reducing the requirement for the high pressure pulses. It is a further object of this invention to teach a method of generating oscillating pressure signals in the drilling fluid thereby facilitating higher data transmission rates. It is a further object and feature of this invention to provide a general method of valving that allows the oscillation amplitude to be set largely independent of the fluid flow rate.